1. Field of the Invention
The present invention relates to a beta-alumina solid electrolyte having excellent properties such as low electrical resistance, high strength and the like, as well as to a process for production thereof. More particularly, the present invention relates to a process for production of beta-alumina solid electrolyte, which can produce a beta-alumina solid electrolyte of having excellent properties without subjecting the materials to calcination (calcination is conducted in conventional processes).
2. Prior Art
Beta-Alumina solid electrolytes have a very high sodium ion conductivity, i.e. a low electrical resistance. Hence, attention is being paid to their use as a diaphragm for a sodium-sulfur battery, and the like.
Of the beta-alumina solid electrolytes, a an MgO-stabilized beta-alumina solid electrolyte has conventionally been produced as follows. That is, the materials, i.e. MgO, a sodium compound and .alpha.-Al.sub.2 O.sub.3 are mixed at an appropriate ratio; the mixture is calcined to obtain beta-alumina; grinding is conducted; the ground material is then granulated, molded into a desired shape and fired to obtain a beta-alumina solid electrolyte.
The reason why the materials are calcined beforehand to obtain beta-alumina, is that when beta-alumina is produced by direct firing without conducting calcination, severe volume expansion in the phase transition from .alpha.-Al.sub.2 O.sub.3 to beta-alumina occurs, making it difficult to obtain a beta-alumina solid electrolyte of uniform quality and high strength.
In the conventional process, however, since the materials are calcined beforehand to obtain beta-alumina, the steps become complex and invite a high cost. Hence, it is strongly desired in production of a sodium-sulfur battery, and the like to shorten the total production steps and produce a beta-alumina solid electrolyte more efficiently.
Further, since the above-mentioned conventional process conducts preliminary calcination (to obtain beta-alumina), grinding, granulation, molding and firing, the crystals constituting the resulting beta-alumina solid electrolyte have high orientation. Moreover, the beta-alumina contains large crystals because crystal growth takes place while the beta-alumina formed by calcination is acting as a nucleus for crystal growth. In addition, many of the large crystals have a shape of large aspect ratio, i.e. a thin and long shape.